1.Field of the Invention
This invention relates to an optical blade such as a shutter blade or a diaphragm blade for intercepting light rays in optical instruments such as photographic cameras, motion picture cameras, holographic cameras like, and more particularly to an optical blade which is driven to close or open a light passage in optical instruments by an electromagnetic force.
2.Description of the Prior Art
It has been desired to reduce the number of parts of a camera to reduce the weight and size thereof. By simplifying the structure of the camera, it is also possible to simplify the operation and lower the manufacturing cost thereof. From the viewpoint of simplification, it is undesirable to drive the various mechanisms in the camera only by mechanical driving means. Therefore, as a means for controlling the shutter mechanism and the diaphragm mechanism it has been proposed and put into practice an electric control means employing an exposure control circuit.
Even in these electrically controlled cameras, however, the control mechanisms are driven by a mechanical force as of a spring. For instance, a spring force is charged by an operation to set a shutter mechanism and locked by a lock member, and then, the lock member is released by means of a solenoid or the like which is controlled by an electric circuit. The shutter mechanism is then driven by the force of the charged spring. These cameras employing a spring force to drive the shutter mechanism or the like require a complicated mechanical driving means. Therefore, the structure and operation of these cameras are still complicated and accordingly the manufacturing cost is still high.
In view of the defects inherent in these cameras, it has been proposed to drive the shutter mechanism or diaghragm mechanism by an electromagnetic force by use of a combination of a permanent magnet and a coil. However, since the driving force of the coil depends upon the number of turns of the windings and the amount of electric current flowing therethrough, the number of turns must be increased to obtain a large driving force with cells which, being small enough to load in the limited space within the camera body, are necessarily of low power, an increase in the number of turns of the coil windings results in an increase in the size of the coil, which is undesirable from the viewpoint of manufacture. Therefore, it is desired that the optical blade electromagnetically driven should have the minimum weight possible. Further, in a case where the shutter blade is to be driven at a great speed, the inertia of the blade is desired to be as small as possible. From this point of view also, the weight of the blade is desired to be as small as possible.
In addition, the conventional optical blade is usually made of metal sheet having the thickness of about 0.04mm and bearing a black mat coating to prevent surface reflection. Such a metal blade has a defect in that the surface coating is apt to be scratched by the sharp edge of an adjacent blade particularly when the blade is used in a diaphragm comprising a number of diaphragm blades. The coating material scratched off falls on the surface of lenses in the camera and degrades the quality of the image. Further, the work of coating the surface of the blade with the black mat material is quite troublesome since the thickness of the blade is as small as several tens of microns. Besides, the metal blade has a large weight since the specific gravity of the metal used for the blade is as large as 7.0. Therefore, the metal blade is not suitable for electromagnetic driving.